MAGNETIC BALANCE TEST – AN EFFECTIVE DIGNOSTIC TOOL FORDETECTION OF SUBTLE FAULTS IN TRANSFORMERS Vishal Mahire, DGM (Testing), EMCO Limited, Thane E-mail id: [email protected] Abstract: Magnetic Balance test is a most commonly used proactive test to detect faults in the core and/or the windings of the transformer at early stage of manufacturing at works. Although considered as the simplest test to conduct, sometimes it is difficult to interpret the results conclusively because of some unpredictable variations in test results. Invariably, for authenticating the analysis of results, we have to conduct some supplementary tests also. Nevertheless, at times we do not find suitable technical explanation for certain strange test results, leaving us clueless about the type of abnormality in the transformer. This paper discusses typical trends of induced voltages in un-excited limbs of transformer, observed during magnetic balance test and the measurement of magnetizing currents. It also attempts to give explanation for the observed trend such as lesser magnetizing current in the centre limb than the extreme limbs, the effect of delta connected windings on distribution of 3-ph magnetizing currents and the combined voltage magnitude in the un-excited limbs in excess of the applied voltage, etc. Through various case studies supported by data analysis, the author shares his extensive testing experiences about magnetic balance and magnetizing current tests and suggests possibilities of leveraging the knowledge for detection of various faults and abnormalities within the windings and magnetic core of the transformer. The case studies present step by step method for analysis of test results to help diagnosis and location of faults. Findings of the paper will help gain an insight in to the subtle quality problems in the windings and core during manufacturing of transformer or the in-service problems developed during operation of the transformers, and enrich reader’s knowledge encouraging optimum deployment of magnetic balance test and the complementary magnetizing current measurements, which are recognized as very effective tests to detect typical problems like: a) b) c) d) Inter-turn shorting; Inter-strand shorting; Any external loops around the core; Abnormal magnetizing current due to unequal turns in winding sections connected in parallel; e) Wrong interleaving joints in windings. The paper discusses several aspects of core and winding configurations with a view to address some of the most sought-after answers on the subject. Keywords: Magnetic balance test, Magnetizing current, Fault diagnosis. Although this test is not specified in any national or international standards. If we excite U phase winding by application of 230 V. and measure the voltages in other two limbs. due to less reluctance path 60 to 90 % flux tries to close loop through adjacent V limb and remaining flux travel through higher reluctance path of limb W (Normally 60 to 90% flux gets linked in V phase and remaining 40 to 10% through extreme phase). As the term suggests. This flux distribution condition is purely dependent on core geometry where reluctance path plays major role in flux distribution. Φ2 = 60 to 90% & Φ3 = 40 to 10 % (When voltage is applied to U ph) 2) Φ2 = Φ1 + Φ3 where. flux Φ1 is produced in U limb. Φ2 α Vv. Magnetic Balance Test: This test is performed by application of single phase voltage to individual limb by turn and the voltages induced in other two limbs are measured as shown in the following diagram. ideal expected conditions are. 1) Φ1 = Φ2 + Φ3 where. Φ1 = 50% & Φ3 = 50 % (When voltage is applied to V ph) 3) Φ3 = Φ2 + Φ1 where. Similarly if we excite V limb. This test is however not applicable in case of 1-ph transformers as 3 wound limbs are not available for measurement of induced voltages. Magnetic balance involves checking of balancing of flux distribution in the magnetic circuit of a 3 phase transformer.Introduction: Magnetic balance test (MBT) and measurement of magnetizing current is the simple test used to detect various faults in windings and core in the transformer. and Φ3 α Vw. When we apply voltage to individual limb. due to similar path of core reluctance of other two limbs. Φ2 = 60 to 90% & Φ1 = 40 to 10 % (When voltage is applied to W ph) Here Φ1 α Vu. distribution of magnetic flux is evident during voltage measurement. yet it is quite popular in India. almost same amount of flux passes through both the extreme limbs. As the voltage is directly proportional to the flux. for eg. . . hence it is recommended to perform this test before resistance measurement test or after proper demagnetization of the core. its large no of turns results in poor V/T in the windings.2%) 71 (28.Refer Table:2) Table:2 Measured Voltage in Volts 2U-2N 226 (100%) 115 (50.8%) 249 (100%) Total Voltage induced in other 2 phases 100.7 (25.5 kV Transformer is given below (Refer Table:1).3%) 226 (100%) Total Voltage induced in other 2 phases 101.5%) 2V-2N 178 (71.8%) 56. 220/11 kV Transformer a) Before DC test (conducted at after. DC components are present in the core.5%) 249 (100%) 178 (71.7 (23.0% 100. here underlined figures are the voltage applied terminals. Due to this magnetization.5%) 2W-2N 71 (28.6 mA W 158.3%) 226 (100%) 111 (49.1%) 172.0% 100. In large rating transformers it is observed that ideal condition of magnetic balance is not achieved. This affects the flux distribution in the core and the results obtained can be confusing.Refer Table:3) Table:3 Measured Voltage in Volts Total Voltage induced Magnetizing in other 2 phases Current 2U-2N 2V-2N 2W-2N Phase 233 (100%) 202 (86.4% U 181 mA 151 (64.A typical example of measurements on LV winding of 45 MVA. if this test is performed from HV winding (normally HV winding is placed away from the core).connection stage.0% Magnetizing Current Phase U 158. Table:1 Measured Voltage in Volts 2U-2N 249 (100%) 125 (50.8%) 233 (100%) 100.0 % 101.9%) 55 (24.8%) 233 (100%) 80.2% V 144 mA 54.5%) 179 (76.4%) 99.3 mA Remnant DC component in Core substantially affect test results of MBT. 21/11. the core gets magnetized.1 (34.7%) 32 (13.5%) 124 (49.4 % Magnetizing Current Phase U 135 mA V 99 mA W 136 mA b) After DC winding resistance measurement test (conducted at pre-tanking stage. A typical example is given below for reference which was performed on a 25MVA.7%) 100.0 mA V 118. If low voltage is applied to HV winding.3 % 100.1%) 2V-2N 2W-2N 174 (76.3% W 260 mA From the foregoing we can say that DC test affects both the test results (voltage distribution as well as magnetizing current trend) substantially.6 (76. Hence it is advisable to perform this test before conducting any DC test on the transformer. in the process of obtaining steady resistance value by nullifying the inductive effect of winding. When winding resistance measurement is performed using DC current source. like in case of Furnace Transformers. Results of the measurements carried out from Tertiary winding of 315MVA. having satisfactorily passed all the performance tests. if MBT is performed from LV side then voltage induced in HV windings may exceed the safe limits due to higher ratios.92%) 112.3%) 224. safety aspect becomes more crucial when this test is performed at manufacturing stage or on transformer in un-tanked condition where HV line leads are in open condition. In such cases the test results measured from LV winding should be considered for analysis. handling of very high currents is also not possible.3 % 100.95 mA During commissioning stage some people perform this test from all windings and at all tap positions.0 % 99.80 (3. every turn of each windings of the particular phase gets proportionate voltage induced in it. Hence it is advisable to conduct this test from the winding closer to the Core in case of large rating transformers.7%) 8.2 (49. Moreover.2(95. If there is any problem with any of the turns of any winding. Although it is advisable to perform this test from LV side there are some exceptions to this concept. in LVs of Furnace and Rectifier Transformers. Experience shows that these transformers.6(3.85 mA 2.4(50. in some extreme cases this voltage even exceeds 30% to 35%. YNa0d11 connected autotransformer are given in Table: 5 as example. 400/220/33 kV. Moreover due to higher voltage induction in HV windings it is as good as performing this test at higher voltage level.which results in development of poor flux and the test results obtained may not be reliable. Sometimes very surprising test results are experienced in case of MBT test from delta connected tertiary windings in large rating autotransformers. 220/66/11 kV System Transformer is given in Table: 4 Table:4 Measured Voltage in Volts 1U-1N 224.23 mA 2. as when we apply voltage to the LV winding.9%) 1W-1N 8.8 % Magnetizing Current Phase U V W 2. Practically there is no need to perform this test from different windings and at different tap positions. In such cases.3(100%) Total Voltage induced in other 2 phases 99. Hence this phenomenon is considered to be a normal in case of autotransformers.4(95.4%) 223. Rectifier Transformers and Transformers with higher voltage ratio like Generator Transformers.83%) 1V-1N 214.7 (100%) 111. There are some cases where as low as 4 to 5% voltage induction is measured in the extreme limb. it will reflect in the test results due to flux choking in that particular limb. In such cases the sum of voltages induced in other two windings exceeds the voltage applied normally by 5% to 10%. An example of measurements on HV winding of 100 MVA. However. .6 (100%) 215. serving in the field satisfactorily for several years. Thus. .0%) 250.Table:5 Measured Voltage in Volts 3U.5%) 230 (100%) 223 (96.8(48.3N 230 (100%) 113 (49.2(100%) 124.7 mA 23.7 mA When studied this phenomenon in case of 11 kV star connected tertiary or auxiliary windings.0(100%) 125.1 mA V 25.1(14. Table:6 Measured Voltage in Volts 3U.3%) 117 (50.0(86.2%) 238. Practically it is seen that.2(99.8%) 3V-3N 222 (96.2(31.0% 100.0(100%) 215.4%) 114.0 mA Above measurements are carried out on the autotransformer at final stage where Transformer is in tanked condition with bushings mounted.3W 249.9 mA 15.8% Magnetizing Current Phase U 106.0%) 37.1%) 87 (37.0(50. Table:7 Measured Voltage in Volts 3U.9%) 3W-3N 88 (38.3(99.0(100%) 35.2(100%) 236.5 mA Measurement of Magnetizing Current: A) Magnetizing current measurement by application of Single phase voltage.3W 239. as an example for a 50/75/100MVA.0(52. 220/132 autotransformer.4%) 3V-3U 238.0(50.0(86.1% 100.5 mA V 61. In contrast to above the results performed on same transformer in un-tanked condition the sum of voltage induced in other two phases is found normal but the magnetizing current is higher by about 30% to 40% as shown in Table:6 below. magnetizing current is more sensitive to the abnormal conditions or faults than voltage.8% 100.8%) 230 (100%) Total Voltage induced in other 2 phases 134.2(14.9 % Magnetizing Current Phase U V W 22.2% 132. similar behavior is observed.6%) 238.8% Magnetizing Current Phase U 36.2%) 3W-3V 75.6 mA W 36.0%) Total Voltage induced in other 2 phases 100.2(33.1%) 80.0 mA W 104.9%) 3V-3U 3W-3V 216.1%) 125. Hence it is always advisable to perform & analyze both the tests simultaneously. where 11kV is the auxiliary star-connected winding provided for works testing purposes.7%) 250.0% 134.1(100%) Total Voltage induced in other 2 phases 131.8 % 100. A case is given below in Table:7. Thus. I = Current. The reluctance (S) of a magnetic path is directly proportional to its length (l) and inversely proportional to its area (A). µ = B / H .t. The reluctance is also inversely proportional to the absolute permeability (µ) of the magnetic material. magnetizing current measured in centre limb is comparatively lesser than that of extreme limbs. This phenomenon can be explained as under1. m. l = length of flux path) . S = (l*H) / (B*A) --------. While performing this test from the outermost winding (mainly HV of large Power Transformers) this variation may be of the order of 82% to 88%.(1) Also. Hence.f.r.m. S = l / ((B/H) * A) i. S = l / (µ*A) The permeability of a magnetic medium is a measure of its ability to support magnetic flux and it is equal to the ratio of flux density (B) to magnetizing force (H) Thus. (Where N = No of turns.e.In normal conditions. This variation w. (H) = (N * I) / l ……. But this variation is quite normal and is a result of lesser reluctance path experienced by flux in centre limb. extreme limbs is of the order of 62 % to 80%. Magnetizing current is directly proportional to the length of the magnetic reluctance path) Thus we can conclude that.4 53 409 322. this method is less helpful in fault diagnosis than single phase method.4 1.5 25.e. However. due to asymmetrical core geometry.220/132/11 KV Dyn11 YNa0d1 HV-110KV TER-11 KV 1.94 100. Table:9 Transformer Rating Vector Voltage Magnetizing current in mA.4 20 MVA. 110/33-11KV 160 MVA. 220/66/11 KV 315 MVA. The trend of measured magnetizing current in 3 ph.5 212.3 136.5 kV YNa0d11 YNynd11 YNynd11 YNa0d11 YNynd11 YNd11 Tert.3 275. Group applied from when 3ph. 220/66/11kV 315 MVA.3 23.5 kV 15. . 220/132/33 kV 100 MVA.1 157. B) Magnetizing current measurement by application of three phase voltage. winding connections within a phase and more importantly on dimensions and layout of the core. 220/66/11kV 100 MVA.53 181. 230/16.05 136 0.1 244.16.3 18.3 159. S α l and S α I Hence I α l (i. It may be helpful to compare the trend of magnetizing current with its historical data. Measurement of magnetizing current by application of 3 phase voltage is normally carried out at site. method is different for different winding connections. 415 V applied U Phase V Phase W Phase Measurements from Delta connected winding 160MVA. more reluctance path is offered to the flux produced in extreme limbs which is mainly responsible for drawing higher magnetizing current than centre limb. This is due to different mutual impedances in 3 phase 3-limb transformers. B & A are constant.4 . Some measurements are tabulated below in Table:9.33 kV TER-11 KV TER-11 KV TER-33 KV TER-11 KV LV . as these mutual impedances are function of number of turns & disposition of windings.Therefore. as N. 400/220/33 kV 160 MVA.6 183 44 289 239. l * ((N * I) / l)) S= (B * A) (N * I) S= -----------.3 245.(2) (B * A) Comparing equation (1) & (2). 5 MVA.8%) 3. one of the extreme phases draws more current than other 2 phases depending upon the delta connection of the winding. a) If measurements are done from star connected winding.66 KV 10. Use of this test for Fault Diagnosis: This test is widely used by Indian end users as a powerful diagnostic tool for checking healthiness of transformer at site post fault occurrence events during operation.3 mA .06 1.0%) 2.9 KV Dyn1 LV-6.W phase draws higher magnetizing current than V & U phase This is to be noted that for delta connected winding also.5/6. Table:10 Measured Voltage in Volts Total Voltage induced Magnetizing in other 2 phases Current 2U-2N 2V-2N 2W-2N Phase 99.6 (1. the trend of 3ph magnetizing current can be summarized as under.8% V 238 (100%) 119.e Iv-mag is lesser by 60-80% of Iu-mag & Iw-mag).68 From the foregoing. 220/66/11kV YNynd11 IV.8%) 23.63 10. 16.6 (49.8 (87.49 Amp 118.47 (1.6 133. 415 V applied Measurements from Star connected winding 10 MVA.8%) 99.99 3. b) If measurements are done from delta connected winding.5 100 MVA.4 6. the trend is similar to the single phase measurement trend (i. YNyn0 connected Power Transformer At pre-connection stage of manufacturing. This implies the need for proper study of results of MBT test along with the complementary magnetizing current measurement tests. when 3ph.0 (100%) 4.9 KV 200. Yd1 & Dyn11 . Manufacturers also use this test to ascertain healthiness of windings at different manufacturing stages. 220/33 kV.5% W 238.92%) 233. This trend is observed at different excitations and confirmed that it remains same at any excitation.4 (97.U phase draws higher magnetizing current than V & W phase Yd11 & Dyn1 .9 mA 99. magnetic section corresponding to V phase required least magnetizing current. Case Study: 1 50 MVA.4 200.0 (50.45%) 23. General understanding about this test is that. Following are some case studies which prove this test as one of the powerful techniques to locate the abnormalities in the transformer windings.Transformer Rating Vector Group Voltage applied from Magnetizing current in mA.81 31.7 1. only inter-turn failures are detected by this test but some case studies discussed below confirm that this test can detect other faults like shorting of parallel conductors and presence of external metallic loop around the core limb. following test results were measured as given in Table:10.3 (100%) 233.220/132/11 KV YNa0d1 IV-132 KV 1.4% U 239.45 4. As observed. 110/33-11KV Dyn11 LV-33 KV 4. but the phasor addition of currents of two phases results into a condition that current in V phase equals the current of one of the extreme phases.83 100 MVA. load losses were found erratic and on higher side (almost 1. To investigate it further. The abnormally high magnetizing current drawn by the limb suggests that there is a inter turn short circuit fault in the winding. 3 Phase Power Transformer Results given in Table:11 are of MBT test conducted from LV side of the transformer before commencing the routine tests on the transformer Table:11 Measured Voltage in Volts 2U-2N 226 (100%) 97(42. However. which indicates there is a closed loop in the V limb which is acting as a localized load. While the transformer withstood the dielectric tests like separate source & induced over voltage withstand successfully.7%) 27(11.2% 100.7%) 227(100%) 30. very poor flux linkage with the winding turns in the limb V. .3 mA V 39. circulating current was checked on the winding discs by clamp meter and gauss meter in which circulating current was sensed in uppermost disc of HV winding. With this it was concluded that there might be an inter-strand shorting in the winding.0% 99. visual inspection of top disc was carried out in which it is found that main lead take off is having sharp bend and this bend touched the 2nd disc.Here voltage trend shows that there is very less voltage is induced in the limb V. Considering this fact above test results could have been accepted. To locate the fault the core coil assembly of the transformer was un-tanked.5%) 130(57. To locate this fault. Based on these findings. Magnetizing current trend shows high current in V limb. all other routine tests were performed on this transformer.8 mA W 49. which showed no abnormality. drawing the load current from the source.9%) 2V-2N 2W-2N 196(86. 1-phase loss measurement was carried out.e. Case Study: 2 40MVA.0 mA Prior to this test. Rated phase voltage applied from LV side to individual phases in which power loss value of U limb was found substantially higher as compared to other two limbs.6% Magnetizing Current Phase U 61. still to confirm the healthiness. YNyn0. In other terms we can say that flux is not able to enter the V limb and getting the path through W limb (as evident from voltage trend). leading to circulating current with consequent increase in the losses in U limb.4 (13. This further crushed it’s insulation during pressing activity and resulted in to short circuiting of the turns. Crimping lug of the lead was cut and discontinuity was checked between the winding strands.3%) 226(100%) 198(87. in which both parallel strands of regulating winding were found shorted at ID side at one of the transpositions. the no. resistance measurement test was carried out at pre-tanking stage. forming a closed loop within the winding.8 times the expected value). i.6%) Total Voltage induced in other 2 phases 100. DGA in oil was conducted before & after routine tests. 132/33 kV. 5 (100%) Total Voltage induced in other 2 phases 100. This type of failure was detected at very early stage by MBT test due to which heavy rework was avoided.8(75. In this case.2) 232.2% 105.4(49.2% 99.8 mA As evident in this case. YNyn0d11 connected System transformer had a inter-strand failure (confirmed by discontinuity test) which was detected at pre-connection stage. but the trend of magnetizing current is abnormal.6(75. In which LV CTC coil of 100 MVA. YNa0d11 connected.3 mA W 50. As magnetizing current in V limb was higher.8 (63.4(100%) 112.8%) 115.9%) 228. there is no significant variation in absolute values of voltages & magnetizing currents. 2 strands in common winding were found shorted at inner transposition due to scissoring action.8(50.9 % 100.5 (37. discontinuity was test performed between individual strands of each winding of V limb.9%) 186.9%) 227. Table:12 Measured Voltage in Volts 3U-3W 226. out of 15 parallel strands of CTC conductor. Table:13 Measured Voltage in Volts 2U-2N 231.6(100%) 171. These types of test results are tricky and confusing and need to be tackled carefully. to analyze it further.6(30.3% 101.1 (26.1%) 46.1 mA .7% Magnetizing Current Phase U 50. For this test temporary delta connection of Tertiary winding was formed. 220/66-33/ 11 kV.9) 86.7%) 169.3%) 3V-3U 3W-3V 70(30.8 (100%) 2V-2N 2W-2N 148.6%) Total Voltage induced in other 2 phases 106.1 mA W 256. 400/220/33 kV.3(100%) 69.6 (100%) 62. 3 Phase Autotransformer Test results given in Table:12 are the results when this test was conducted from Tertiary side of the transformer at pre-connection stage.8% Magnetizing Current Phase U 167.9 (73.5 (80.4%) 171.8 mA V 90. Similarly one more case of inter-strand shorting gave following test results tabulated in Table:13.5%) 232.Case Study: 3 315 MVA.1(19.9 mA V 140. Difference in the magnitude of leakage flux linkage in different strands produces different voltages in the strands of the conductor. This is already discussed in Case Study-. From the above findings it is quite evident that although the voltage distribution does not get affected by such fault. Since no problem in the winding was evident. Further.7%) 201.6% 102.4% Magnetizing Current Phase U V W 565 mA 117 mA 123 mA As there was no abnormality at pre-connection stage.4%) 13(5. 11 kV / 2 x 2 x 0. which may almost twice the expected value of no-load losses.The above Case Studies-2 & 3 give a different dimension to the pre-conceived notion that inter strand shorting can not be detected by this test and shorting of parallel conductors does not affect the performance of the transformer.5(94.1(19. During core inspection it was discovered that a core banding metallic strap was . In case of shorting of any of the strands at different potentials due to insulation damage would nullify the effect of transposition and cause flow of circulating current within the shorted loop of strands. different potential would be induced in these strands. yet the trend of magnetizing current is influenced substantially. possibility of direct inter-turn shorting or wrong interleaved joint was ruled out.68%) 1V-1W 1W-1U 184(80.2 Case study : 4 2 x 8789 / 2 x 2 x 6214 KVA.6(88.3986 kV. Circulating currents measured in the winding with the use of guass meter and clamp on milli-ammeter did not show any abnormality. This phenomenon can be explained as under. The diagram below illustrates the phenomenon.0%) 229(100%) 45. 3 Phase Rectifier transformer Following are the test results when this test was conducted from HV side of the rectifier transformer at post-connection stage. The alternating leakage flux linkage experienced by different strands being different. (Refer Table:14) Table:14 Measured Voltage in Volts 1U-1V 230(100%) 33(14. Say.6(100%) Total Voltage induced in other 2 phases 99. ‘Φ’ is the flux linkage with conductor at position 3. This would eventually reflect in to higher magnetizing current.4% 100. we observe abnormally higher no-load loss as a result of the circulating currents.6%) 216. when the transformer with this type of defect is charged at the rated voltage for noload loss measurements.03%) 228. ‘Φ/2’ is with middle conductor at position 2 & ‘0’ is of the conductor at position 3. decided to carry out the visual inspection of the core thoroughly. phase.5%) 159(74. . While checking the circulating current in the shorting leads of HV Regulating winding (comprising 4 parallel sections) circulating current was observed. which is a clear indication of one interleaved joint made wrongly (i. 1 Phase Traction transformer As discussed earlier. in single phase transformers. To rectify this problem this extra one turn was removed from the winding and MBT was repeated after all connections which found normal.4(12. ratio test between HV and individual step of Regulating winding was performed in which one turn was found more than specified in one of the steps. which had resulted in higher magnetizing current in Wphase.forming a closed loop around the core. To rule out possibility of shorting of bottom Static End Ring (SER).3(25. which indicated the circulating current in bottom most disc. 3 Phase System transformer Following are the test results when this test was conducted from Tertiary side of the transformer at post-connection stage during manufacturing of the transformer. Test results indicated that out of two strands.5%) Test results indicated problem in the windings of W. (Refer Table:15) Table:15 Measured Voltage in Volts Total Voltage induced Magnetizing in other 2 phases Current 3U-3W 3V-3U 3W-3V Phase 99. dummy turn brazed to dummy turn). presence of circulating current in the winding checked with the help of clamp on milli-ammeter.6 MVA. thus forming a short circuited turn. Case study 6: 21. YNyn0d11 connected. Parallel shorting leads were cut apart and MBT was repeated with satisfactory results.8%) 130 mA 100. to diagnose the fault. Also magnetizing current was found 26mA from HV side as against 4mA value measured in earlier units.1%) 12. 220/66/ 11 kV. To locate the fault.e. SER was physically checked. hence it becomes essential to compare magnetizing currents of such transformers with similar units. All cross-overs in that zone were checked for any damage of conductor paper insulation due to scissoring action and these were also found intact. 132/27 kV. Further.0% W 213. While testing this transformer at pre-connection stage phase angle was found higher during ratio measurement.3(100%) 750 mA 54. only magnetizing current can be measured. which formed a metallic loop over the top yoke between U and V limbs. which was found healthy.8%) 100.0% V 213(100%) 185.4(5. This extra one turn was causing the potential difference between the parallel coils. The operator had missed to provide the isolating wooden block to one metal strap. Later. one strand has less number of turns compared to other.4(94.7%) 27. resulting in circulating current.9% U 213(100%) 140 mA 200. Physically this fault was located at the winding start and could be repaired by interchanging the main and dummy conductor at the interleaved joint. Case study: 5 100 MVA. Ratio measurement was carried out on individual strand of the HV coil which is interleaved disc winding.7(8. Kulkarni Acknowledgements: Author is thankful to the EMCO Management for granting permission to publish this paper. 400/√ 3 / 220/√3 / 33 kV. While inspecting those discs it is observed that while making interleaved joint in the winding. . 1 Phase Autotransformer At pre-connection stage. e) Wrong interleaving joints in windings. This turn was acting as a localized load to the supply source and drawing high current. In series winding between disc no. proves to be a strong diagnostic test to detect Faults like a) Inter-turn shorting. circulating current checked through out the series winding axially.Case study 7: 167 MVA. d) Abnormal magnetizing current due to unequal turns in winding sections connected in parallel. b) Inter-strand shorting. To confirm further with the help of clamp on type milli-ammeter. 61 & 62 high circulating current was observed. unscrupulously the winder had brazed dummy conductor to dummy conductor. V. c) Any external loops around the core. which resulted in a short circuited turn. Conclusion: Magnetic Balance test when supplemented with measurement of magnetizing current. high magnetizing current of the order of 10A was measured at 230V when tested from Tertiary winding side as against 40mA measured earlier on similar units. Reference books: 1) Electronic Circuits – Fundamental & Applications by Mike Tooley 2) Transformer Engineering – by S. it is recommended to perform this test before resistance measurement test or after proper demagnetization of the core as remnant DC component in Core substantially affect test results of MBT. To obtain accuracy in the test results.
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